Benjamin van Wyk de Vries

Sector collapse forming at Casita volcano, Nicaragua

Catastrophic sector collapse occurs when a volcano becomes structurally unable to support its own load. One process particularly capable of weakening the edifice is hydrothermal activity. It can produce high pore pressures and alter strong rock to clays. Alteration can extend progressively over long periods (>100 yr), allowing deformation to develop slowly before collapse. An important finding is that structures produced by such deformation are recognizable and could permit collapse prediction. We present the case of Casita, Nicaragua, where hydrothermal activity has been weakening the edifice core, causing flank spreading, altering original constructional shape, and steepening flank slopes. One side is slumping outward, producing a crescentic scar with a basal bulge. We identify this feature as the site of a potential sector collapse, with conditions ripe for failure.

The role of basement in volcano deformation

Abstract Growing volcanoes load their basements, causing isostatic flexure, compaction and brittle and ductile deformation. Deformation of basement in turn exerts stress onto the edifice, which also responds by deforming. This deformation will be gradual or catastrophic, depending on the nature of the cone, and the properties of the underlying rocks. The physical properties of substratum, most especially the viscosity, can vary greatly. Thus a substratum of lava will support a volcano to a greater extent than fine clastic rocks. Compaction, crustal flexure, and sagging (a result of viscous flow of substratum) create a depression below the volcano. In the cone this induces compression, which increases with increasing sagging. However over long time spans, or in low viscosity substratum, sagging generates volcano spreading, where elastic compression is relaxed. The role of basement in volcano stability at four Nicaraguan stratocones 1.5–2 km high, constructed of similar materials on three types of basement has been studied. San Cristobal volcano has a volcanic basement of lava and ignimbrite, which has sagged by small amounts. Mombacho volcano has a basement of volcanic and marine strata. Sagging occurs, but spreading is slow. Compressive stresses have been released in the cone by thrust faulting. These structures have probably been exploited by three sector collapses. Concepción and Maderas volcanoes rest on sedimentary strata of weak mudstones, and large amounts of sagging and spreading have occurred. The study shows that spreading promotes gradual deformation in the volcano, because slopes are reduced and stresses relaxed. Catastrophic failure is favoured when compression is dominant, because stresses are large enough to fracture rock and produce decollements. Fracturing also promotes hydrothermal circulation, which further weakens the construct.

Structural analysis and analogue modeling of the kinematics and dynamics of rockslide avalanches

We present a structural analysis of subaerial natural and analogue rockslide avalanches. Such deposits often have well-developed faults, folds, and hummocks. These structures can be used to determine the kinematics and dynamics of emplacement. Large-scaleterrestrial rockslide avalanches show large runout distances compared to their fall height. Most attempts to explain this phenomenon invoke uidizing mechanisms or lubricating agents to reduce forces opposed to momentum, especially at the base. However, the properties and mechanics of low friction are still poorly understood. Any model for motion and emplacement must integrate geometric, morphologic, and structural features, all crucial in constraining kinematics and dynamics. Here we first examine the morphological and structural features displayed by 13 natural rockslide avalanche deposits; we then use simple and well-constrained analogue models involving the slide of stratified granular material down smooth, curved ramps. These differ from previous analogue models in that we concentrate on observing the structures produced by brittle deformation and use a low-friction sliding surface. Models show that variations in the sliding surface curvature, lateral profile, roughness, and modifications in material cohesion can successfully reproduce the majority of rockslide-avalanche deposit features. After discussing the geometrical and dynamic similarity between experiments and natural examples, we propose a model for structure formation and a fourfold classification based on model and natural deposit morphology and dynamics: hummocky, nonhummocky, dominantly extensional, and dominantly compressional rockslide avalanches. The models require a brittle core and surface that spreads and contracts by adjustment on large numbers of faults that bottom into a low-friction decollement layer. Spreading is accommodated by normal and strike-slip faults, while on deceleration, thrust faulting generates thickening. To be realistic, any physical predictive model must take into account these fundamental kinematic and structural aspects.

Morphometry and evolution of arc volcanoes

Volcanoes change shape as they grow through eruption, intrusion, erosion, and deforma- tion. To study volcano shape evolution we apply a comprehensive morphometric analysis to two contrasting arcs, Central America and the southern Central Andes. Using Shuttle Radar Topography Mission (SRTM) digital elevation models, we compute and defi ne parameters for plan (ellipticity, irregularity) and profi le (height/width, summit/basal width, slope) shape, as well as size (height, width, volume). We classify volcanoes as cones, sub-cones, and massifs, and recognize several evolutionary trends. Many cones grow to a critical height (~1200 m) and volume (~10 km 3 ), after which most widen into sub-cones or massifs, but some grow into large cones. Large cones undergo sector collapse and/or gravitational spreading, without sig- nifi cant morphometry change. Other smaller cones evolve by vent migration to elliptical sub- cones and massifs before reaching the critical height. The evolutionary trends can be related to magma fl ux, edifi ce strength, structure, and tectonics. In particular, trends may be controlled by two balancing factors: magma pressure versus lithostatic pressure, and conduit resistance versus edifi ce resistance. Morphometric analysis allows for the long-term state of individual or volcano groups to be assessed. Morphological trends can be integrated with geological, geophysical, and geochemical data to better defi ne volcano evolution models.

Volcano spreading controlled by dipping substrata

Most volcanoes grow on slopes, and some tend to fail catastrophically on the downslope side. Many volcanoes also deform by volcano spreading, which may lead to failure. We look at the effect of dipping substrata on the potential for spreading and collapse with analogue models. The dip is found to strongly control the spreading style, rate, and direction. A distinct change from purely radial spreading occurs even with small (<1°) substrata tilt. Structures on the cone and surrounding area are modified according to the underlying dip direction. Spreading becomes concentrated on downslope sectors, where movement is predominantly in the dip direction. The degree of structural realignment is a function of the slope angle. Our models are applied to previously known and new examples of volcano spreading. The effect of dipping substrata in confining spreading to sectors increases the potential for deep-seated sector collapses. This finding provides a mechanism for failure on the downslope side. The incorporation of substrata can create very large volume collapses.

Experiments on vertical basement fault reactivation below volcanoes

Scaled modeling has been conducted to study the role played by the reactivation of vertical basement faults in the destabilization process of overlying volcanoes. Results show that basement fault reactivation induces the formation of faults within the volcanic edifice. These faults delimite a central block which is extruded from between the two undeformed lateral parts of the edifice. Collapse of this central block can occur when heterogeneous cones are used in experiments, revealing that mechanical interfaces are of paramount importance in triggering rockslide avalanches. Collapse structures may display horseshoe shapes in map view and are then surprisingly similar to avalanche scars resulting from failure created by magmatic intrusions in natural volcanoes. Large-scale destabilisation of half the surface of the cone may also occur in a single event depending on the position of the reactivated basement fault below the edifice. It is emphasized that the process under consideration may occur on a dormant volcano as well, provided that a fault is reactivated below it.

Relationships between volcano gravitational spreading and magma intrusion

Volcano spreading, with its characteristic sector grabens, is caused by outward flow of weak substrata due to gravitational loading. This process is now known to affect many present-day edifices. A volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops. Such intrusions are clearly observed in ancient eroded volcanoes, like the Scottish Palaeocene centres, or in geophysical studies such as in La Réunion, or inferred from large calderas, such as in Hawaii, the Canaries or Galapagos volcanoes. Volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice. We explore the coupling and interactions between these two processes. We use scaled analogue models, where an intrusive complex made of Golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer. We model specifically the large intrusive complex growth and do not model small-scale and short-lived events, such as dyke intrusion, that develop above the intrusive complex. The models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading. The brittle substratum strongly controls the deformation style, the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer-related gravitational spreading. In the models, intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone (i.e. grabens, folds, en échelon fractures). Therefore, simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading, except when the intrusive complex is eccentric from the volcano centre. However, the displacement fields obtained for (1) a solely gravitational spreading volcano and for (2) a gravitational spreading volcano with a growing and spreading intrusive complex are very different. Consequently, deformation fields (like those obtained from geodetic monitoring) can give a strong indication of the presence of a spreading intrusive complex. We compare the models with field observations and geophysical evidence on active volcanoes such as La Réunion Island (Indian Ocean), Ometepe Island (Nicaragua) and eroded volcanic remnants such as Ardnamurchan (Scotland) and suggest that a combination between gravitational and intrusive complex spreading has been active.

Unzipping Long Valley: An explanation for vent migration patterns during an elliptical ring fracture eruption

Long Valley caldera, California, formed during the cataclysmic Pleistocene eruption of the Bishop Tuff. Previous stratigraphic and petrologic studies of this eruption deciphered an intriguing pattern of vent migration, thought to mirror the lateral propagation (“unzipping”) of magma-tapping ring fractures during caldera collapse. From scaled analog models, we show that this unzipping pattern was intrinsically related to the high plan-view ellipticity of the precollapse magma chamber roof. We also provide a first-order kinematic explanation for the systematic location of initial elliptical roof failure and for the lateral propagation of highly elliptical ring fractures.

Structure and dynamics of surface uplift induced by incremental sill emplacement

Shallow-level sill emplacement can uplift Earth’s surface via forced folding, providing insight into the location and size of potential volcanic eruptions. Linking the structure and dynamics of ground deformation to sill intrusion is thus critical in volcanic hazard assessment. This is challenging, however, because (1) active intrusions cannot be directly observed, meaning that we rely on transient host-rock deformation patterns to model their structure; and (2) where ancient sill-fold structure can be observed, magmatism and deformation has long since ceased. To address this problem, we combine structural and dynamic analyses of the Alu dome, Ethiopia, a 3.5-km-long, 346-m-high, elliptical dome of outward-dipping, tilted lava flows cross-cut by a series of normal faults. Vents distributed around Alu feed lava flows of different ages that radiate out from or deflect around its periphery. These observations, coupled with the absence of bounding faults or a central vent, imply that Alu is not a horst or a volcano, as previously thought, but is instead a forced fold. Interferometric synthetic aperture radar data captured a dynamic growth phase of Alu during a nearby eruption in A.D. 2008, with periods of uplift and subsidence previously attributed to intrusion of a tabular sill at 1 km depth. To localize volcanism beyond its periphery, we contend that Alu is the first forced fold to be recognized to be developing above an incrementally emplaced saucer-shaped sill, as opposed to a tabular sill or laccolith.

Endogenous and exogenous growth of the monogenetic Lemptégy volcano, Chaîne des Puys, France

The monogenetic Lemptegy volcano in the Chaine des Puys (Auvergne, France) was quarried from 1946 to 2007 and offers the possibility to study scoria cone architecture and evolution. This volcano was originally 50–80 m high, but scoria excavation has resulted in a 50-m-deep hole. Beginning in the 1980s, extraction was carried out with the advice of volcanologists so that Lemptegy’s shallow plumbing system and three-dimensional stratigraphy have been preserved. Detailed mapping enabled key stratigraphic units to be distinguished and the constructional phases to be reconstructed. The emplacement and evolution of the shallow plumbing system have also been unraveled. The growth of this monogenetic scoria cone included two temporally well-separated eruptions from closely spaced vents. The activity included Hawaiian, Strombolian and Vulcanian explosions, lava effusion, cryptodome and dome formation, partial collapse, satellite vent formation, eruptive pauses, and intrusion emplacement with consequent uplift. The cone shape, structure, and hence the local stress field, plumbing system, and thermal state were continuously changing, which in turn influenced the eruptive style and location. The plumbing system morphology and microtectonic structures both record local stress field and magmatic flow direction changes. Lemptegy volcano’s internal architecture, stratigraphy, and evolution show how complex a monogenetic volcano can be.